What should be paid attention to when cutting, grinding and polishing the optical lens substrate made of quartz stone?
Quartz stone is often used for high-precision optical lens substrates (such as astronomical telescopes and laser optical systems) because of its high hardness (Mohs hardness 7), low expansion coefficient and excellent optical transmittance. The process of cutting, grinding and polishing requires extremely high precision and surface quality (such as scratches, roughness and flatness), so it is necessary to strictly control process parameters and operation specifications. The specific precautions are as follows:
1. Cutting stage: ensure contour accuracy and reduce initial damage.
Cutting is to process the quartz stone blank into a substrate prototype close to the size of the finished product, and it is necessary to avoid cracks, edge collapse or stress concentration, so as to lay the foundation for the subsequent process.
Selection of cutting mode and parameter control
Diamond wire cutting (instead of traditional grinding wheel cutting) is preferred: the wire diameter is 0.1-0.3mm, and high-precision cutting can be realized through the grinding action of diamond particles, which can reduce edge collapse (controlled within 5μm) and the roughness ra of the cutting surface is ≤ 1 μ m.
Key parameters:
Linear speed: 8-15m/s (if the speed is too high, the wire saw will be overheated and broken; if it is too low, the cutting efficiency will be low and the surface will be rough);
Feed speed: 0.5-2mm/min (adjusted according to the thickness of quartz stone, and the thick material needs to avoid cracks at low speed);
Cooling and lubrication: use special cutting fluid (such as polyethylene glycol-based coolant) to continuously wash the cutting area, reduce the temperature (to avoid microcracks of quartz stone due to thermal shock) and take away the debris.
Clamping and positioning accuracy
Adopt vacuum adsorption or elastic fixture: to avoid stress damage caused by hard contact, the fixture positioning benchmark should be strictly aligned with the cutting path (positioning error ≤0.01mm), and the dimensional tolerance (such as side length and thickness) of the cut substrate should be controlled within 0.02 mm..
Inspection after cutting: use a microscope (100 times) to check whether there are cracks and notches on the edge, and use a micrometer to measure the thickness uniformity. Unqualified parts should be removed (to avoid amplification defects in subsequent processing).
Second, grinding stage: eliminate cutting marks and control flatness and parallelism.
The core of grinding (rough grinding → fine grinding) is to remove the damaged layer (usually 5-10μm deep) on the cutting surface, and at the same time ensure the flatness (≤λ/10, λ is 632.8nm laser wavelength) and parallelism (≤ 3 ") of the substrate.
Abrasive selection and particle size gradient
Rough grinding: use diamond grinding wheel or grinding blade (particle size 80#-200#) to remove macro unevenness caused by cutting, and grind off the thickness of 0.1-0.3mm (which needs to exceed the depth of damaged layer);
Fine grinding: use diamond micropowder (3-10μm) or silicon carbide abrasive, with cast iron or resin grinding disc, to reduce the surface roughness to Ra≤0.05μm, and control the flatness error within 0.5μm/100mm.
Key points of process control
Uniform pressure: pneumatic or hydraulic loading device is used to ensure uniform pressure (0.1-0.3MPa) of the grinding disc on the substrate, so as to avoid deformation or thickness deviation of the substrate caused by excessive local stress;
Rotation speed matching: the rotation speed of the grinding disc is 300-500rpm during rough grinding, and it is reduced to 100-200rpm during fine grinding (low speed can reduce the scratches formed when the abrasive is embedded in the surface);
Cooling and cleaning: rinse with deionized water continuously, and remove the abrasive chips in time (quartz chips have high hardness, which will lead to secondary scratches if they remain). After fine grinding, ultrasonic cleaning (power 300-500W, time 5-10min) is needed to remove the abrasive particles attached to the surface.
3. Polishing stage: realizing ultra-smooth surface and meeting optical performance requirements.
The goal of polishing is to obtain an ultra-smooth surface (roughness Ra≤0.5nm) without scratches and defects, while maintaining flatness and dimensional accuracy, which is the key process to determine the performance of optical lens substrates.
Polishing materials and tools
Polishing agent: Colloidal silica (SiO₂) or cerium oxide (CeO₂) polishing solution is commonly used, with particle size of 0.01-0.1μm and concentration of 5%-10% (too high concentration will easily lead to surface residue, too low will lead to low efficiency);
Polishing pad: polyurethane or felt pad (Shore A 60-80 hardness) should be selected, and the surface should be smooth. Before use, it should be soaked with deionized water to avoid uneven polishing due to uneven local hardness.
Optimization of polishing parameters
Pressure: 0.05-0.15MPa (too high pressure will cause the substrate to heat and deform, and too low pressure will lead to low polishing efficiency);
Rotating speed: 50-150rpm (low-speed polishing can reduce uneven distribution of polishing solution caused by centrifugal force);
Time: adjust according to the surface state, usually 30-60min (the end point needs to be judged by real-time detection to avoid over-polishing, which will lead to size deviation).
Pollution and defect control
Clean environment: polishing should be carried out in a Class 1000 clean room (particles ≥0.5μm per cubic foot ≤1000), and operators should wear dust-free clothes to avoid the formation of impurity defects on the surface of dust;
Purity of polishing solution: use electronic-grade deionized water (resistivity ≥ 18.2 m Ω cm) to prepare polishing solution to avoid the decrease of optical transmittance caused by metal ions (such as Fe³⁺ and Cu) pollution;
End point detection: atomic force microscope (AFM) is used to detect the surface roughness, and laser interferometer is used to detect the flatness to ensure that there are no scratches, pits and haze (haze value ≤0.1%).
Fourth, the general precautions in the whole process
stress control
Quartz stone is brittle, so it is necessary to avoid violent vibration (for example, the equipment foundation needs shock-proof treatment) and sudden temperature change (the ambient temperature is controlled at 20 1℃ and the humidity is 40%-60%) during processing to prevent the substrate from cracking due to thermal stress;
Special anti-static tray is needed for inter-process transfer to avoid new damage caused by collision or friction between substrates.
Detection and traceability
100% inspection after each process: measure the size and edge quality after cutting, measure the flatness and roughness after polishing, and measure the surface defects and optical properties (such as transmittance and refractive index uniformity) after polishing;
Record key parameters (such as cutting linear speed, polishing pressure and time), and establish a traceability system, which is convenient for analyzing the causes of defects (such as scratches on a batch of substrates, which can be traced back to whether the polishing pad is worn).
Tool maintenance
The diamond wire saw, grinding disc and polishing pad need to be inspected regularly: if the wire saw is broken, it needs to be replaced immediately; if there are grooves on the surface of the grinding disc, it needs to be reshaped; after the polishing pad is used for 10-20 hours, it needs to be turned over or replaced (to avoid polishing deviation caused by uneven wear).
summary
The processing core of quartz optical lens substrate is * * "reducing damage, controlling precision and ensuring cleanliness" * *: initial defects should be avoided in cutting stage, flatness should be accurately controlled in grinding stage, ultra-smooth surface should be realized in polishing stage, and the environment, tools and parameters should be strictly controlled in the whole process to meet the requirements of optical system for "zero defects" and high precision of substrate.
